When you first put the cup in, makes ure there is no air in it. The co2 will fill it and the water flowing under the cup will absorb the co2. This method works for a small tank. You can speed up the process by running a current with a powerhead under the cup so that the water/gas exchange takes place quicker. This methid probably won't work very well for anything larger than a 15 gal tank. You do not put a hole in the cup. The idea is to trap the gas long enough so that it'll get dissolved into the water.